Illumination system for electron beam lithography tool

Details Illumination system for electron beam lithography tool Illumination system for electron beam lithography tool

2000-05-30

2001-12-25

Berman, Jack (Department: 2881)

Radiant energy

Irradiation of objects or material

Irradiation of semiconductor devices

C250S42300F, C313S310000, C313S412000, C355S053000

Reexamination Certificate

active

06333508

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an illumination system for an electron beam lithography apparatus used for the manufacture of semiconductor integrated circuits and a lithographic apparatus having such an illumination system.
BACKGROUND OF THE INVENTION
Electron beam exposure tools have been used for lithography in semiconductor processing for more than two decades. The first e-beam exposure tools were based on the flying spot concept of a highly focused beam, raster scanned over the object plane. The electron beam is modulated as it scans so that the beam itself generates the lithographic pattern. These tools have been widely used for high precision tasks, such as lithographic mask making, but the raster scan mode is found to be too slow to enable the high throughput required in semiconductor wafer processing. The electron source in this equipment is similar to that used in electron microscopes, i.e., a high brightness source focused to a small spot beam.
More recently, a new electron beam exposure tool was developed based on the SCALPEL (SCattering with Angular Limitation Projection Electron-beam Lithography) technique. In this tool, a wide area electron beam is projected through a lithographic mask onto the object plane. Since relatively large areas of a semiconductor wafer (e.g., 1 mm
2
) can be exposed at a time, throughput is acceptable. The high resolution of this tool makes it attractive for ultra fine line lithography, i.e., sub-micron.
The requirements for the electron beam source in SCALPEL exposure tools differ significantly from those of a conventional focused beam exposure tool, or a conventional TEM or SEM. While high resolution imaging is still a primary goal, this must be achieved at relatively high (10-100 &mgr;A) gun currents in order to realize economic wafer throughput. The axial brightness required is relatively low, e.g., 10
2
to 10
4
Acm
−2
sr
−1
, as compared with a value of 10
6
to 10
9
Acm
−2
sr
−1
for a typical focused beam source. However, the beam flux over the larger area must be highly uniform to obtain the required lithographic dose latitude and CD control.
A formidable hurdle in the development of SCALPEL tools was the development of an electron source that provides uniform electron flux over a relatively large area, has relatively low brightness, and high emittance, defined as D*&agr; micron*milliradian, where D is beam diameter, and &agr; is divergence angle. Conventional, state-of-the-art electron beam sources generate beams having an emittance in the 0.1-400 micron*milliradian range, while SCALPEL-like tools require emittance in the 1000 to 5000 micron*milliradian range.
Further, conventional SCALPEL illumination system designs have been either Gaussian gun-based or grid-controlled gun-based. A common drawback of both types is that beam emittance depends on actual Wehnelt bias, which couples beam current control with inevitable emittance changes. From a system viewpoint, independent control of the beam current and beam emittance is much more beneficial.
SUMMARY OF THE INVENTION
The present invention is directed to a charged particle illumination system component for an electron beam exposure tool and an electron beam exposure tool that provides independent emittance control by placing a lens array, which acts as an “emittance controller”, in the illumination system component. In one embodiment, a conductive mesh under negative bias is placed in the SCALPEL lithography tool kept at ground potential, forming a multitude of microlenses resembling an optical “fly's eye” lens. The mesh forms an array of electrostatic lenslets that splits an incoming solid electron beam into a multitude of subbeams, such that the outgoing beam emittance is different from the incoming beam emittance, while beam total current remains unchanged. The mesh enables beam emittance control without affecting beam current. In another embodiment, the illumination system component is an electron gun. In yet another embodiment, the illumination system component is a liner tube, connectable to a conventional electron gun.
The optical effect of a mesh grid may be described in geometrical terms: each opening in the mesh acts as a microlens, or lenslet, creating its own virtual source, or cross-over, having diameter d, on one side of the mesh grid. Each individual subbeam takes up geometrical space close to L, where L equals the mesh pitch. The beam emittance ratio after the mesh grid to the one created by the electron gun, equals
r=
(
L/d
)
2
.
In another embodiment of the present invention, a mesh grid includes multiple (for example, two, three, or more) meshes. In an odd numbered configuration (greater than one), the outward two meshes may have a curved shape; such a lens would enable beam emittance control and also reduce spherical aberration.
In another embodiment of the present invention, the lens array is a continuous lens made of foil.
In another aspect of the invention, the beam controller provides an array of quadrupole electrostatic lenslets. In a preferred embodiment, the quadrupole electrostatic lenslets are formed by three planar meshes that are in spaced parallel relation. These meshes are each formed by a plurality of parallel wires
The invention is also directed to a method of controlling the emittance of a charged particle beam by passing the beam through an array of quadrupole electrostatic field patterns and a method of producing semiconductor devices including passing the beam through an array of quadrupole electrostatic field patterns.
Other objects and advantages of the invention will be appreciated from the following description of the drawings and the appended claims.


REFERENCES:
patent: 5258246 (1993-11-01), Berger et al.
patent: 5260151 (1993-11-01), Berger et al.
“Scanning Thermionic Emission Imaging of Cathode Surfaces” by Sewell et al, SPIE, vol. 3777, Jul. 1999, pp. 125-132.*
Jansen, G.H., “Coulomb interactions in particle beams,” published in Nuclear Instrumeents and Methods in Physics Research A298 (Apr. 1990) 496-504 North Holland.
Katsap et al, Mesh-equipped Wehnelt source for SCALPEL (TM), Proc. of SPIE, Conference on Charged Particle Optics IV, Denver, Colorado, Jul. 1999, SPIE vol. 3777 pp. 75-81.
Mkrtchyan et al, “Stochastic scattering in charged particle projection systems: A nearest neighbor approach,” J. Appl. Phys. 78 (12), Dec. 15, 1995, pp. 6888-6902.
Jansen, G.H., “Interactions in particle beams”, J. Vac. Sci. Technol. B 6 (6), Nov./Dec. 1988, pp1977-1983.

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